The goal is to develop a cell-based technology that will allow engineering vascularized tissues in vivo by using postnatal progenitor cells that can be obtained from patients by non-invasive means. In particular, this proposal will focus on the combined use of bone marrow-derived mesenchymal progenitor cells (MPCs) and human blood-derived endothelial progenitor cells (EPCs) to form adipose tissue in vivo. The concept to test is whether implants containing highly purified and defined MPCs and EPCs will result in specialized vascularized tissues upon implantation in vivo. We hypothesize that for this to happen, 1) both MPCs and EPCs need first to create a vascular network that will allow blood perfusion through the implants; and 2) this vascularization will promote MPCs that have not assembled into the vascular network to undergo appropriate tissue development by adopting the phenotype ofthe surrounding tissue at the site of implantation. In this proposal, we will determine key parameters to accelerate the vasculogenic process to a time frame of 24-48 hours using the in vivo model we have established with human EPCs and MPCs. Next, we will determine whether the adipocytes found in the implants are human in origin or whether they are recruited into the implants from the host as part of an endogenous tissue repair process once a functional vascular network is provided. With this information, we will implement strategies to enhance the process of in vivo differentiation with the aim of having fully functional tissues. Finally, we will test this cell-based technology by evaluating whether we can create long-lasting adipose tissue pads that present low volume reduction over time. The expenmental approaches to address these aims include: 1) in vivo imaging of blood perfusion in the implants using luciferase-based bioluminescence 2) GFP labeling of implanted cells to track their specific contribution to the resulting tissues; 3) confocal microscopy to evaluate implants through immunofluorescence staining. We envision this two-cell, two-step system as an enabling technology that can be applied to many different tissues wherein functional blood vessels are essential.